1. Field of the Invention
This invention relates to an evaporative fuel-processing system for internal combustion engines, which purges evaporative fuel generated in the fuel tank into the intake system of the engine, and more particularly to an evaporative fuel-processing system of this kind, which has a function of determining whether or not a leak occurs in the system.
2. Prior Art
There is conventionally known an evaporative fuel-processing system for internal combustion engines, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 5-79408, as shown in FIG. 1. The known evaporative fuel-processing system is comprised of a fuel tank 101, a canister 106 accommodating an adsorbent for adsorbing evaporative fuel generated in the fuel tank 101, an air-introducing passage 108a extending from the canister 106 and opening into the atmosphere, a vent shut valve 108 arranged across the air-introducing passage 108a, a charging passage 102 connecting between the canister 106 and the fuel tank 101, a two-way valve 104 arranged across the charging passage 102, a passage 102a connected to the charging passage 102, which bypasses the two-way valve 104, a bypass valve 105 arranged across the passage 102a, a purging passage 107 connecting between the canister 106 and the intake system of the engine, and a purge control valve 109 arranged across the purging passage 107.
The known evaporative fuel-processing system functions such that evaporative fuel generated in the fuel tank 101 is stored in the canister 106 and evaporative fuel is purged from the canister 106 into the intake system when the engine is in a predetermined condition. In the system, the vent shut valve 108, the bypass valve 105, and the purge control valve 109 are each formed by an electromagnetic valve, which has its valving operation controlled by a control unit 111.
The vent shut valve 108 and the bypass valve 105 are provided for carrying out leak-checking, i.e. determining whether there is a leak from the system. Therefore, normally, i.e. when leak-checking is not carried out, the vent shut valve 108 is kept open and the bypass valve 105 is kept closed. On the other hand, when leak-checking is carried out, the vent shut valve 108 is closed and the bypass valve 105 and the purge control valve 109 are opened to allow negative pressure from the intake system of the engine to be introduced into the canister 106 and the fuel tank 101. When the canister 106 and the fuel tank 101 have thus been brought into a predetermined negatively pressurized state, leak-checking is carried out based on an output from a pressure sensor 110 inserted into the charging passage 102.
In the known evaporative fuel-processing system, however, the vent shut valve 108 and the bypass valve 105 are formed by electromagnetic valves which are generally expensive, and therefore the manufacturing cost is high.